A Study On Mechanism Of Fracturing Fluid Flowback During Fracture Closure And Design Of Flowback Procedures

In this thesis, theory of fracturing fluid transportation, proppant movement and facture configuration development during fracturing fluid flowback were studied, and the main purpose of the study was to provide the design of flowback procedures with necessary theoretic basis and guidance schemes.Firstly, according to theory of fluid flow in porous media, mechanism of leakoff of fracturing fluid during fracture closure was analyzed and a 2D dynamic leakoff model of non-Newtonian fracturing fluid was established. Numerical method was employed to solve this model and the simulation results were compared with that of traditional ID model. It showed that the 2D model was more applicable to fields because of its consideration of fracturing fluid leakoff in the direction that is parallel to the fracture face and the leakoff rate calculated by 2D model was more reasonable than by traditional ID model.Secondly, after the leakoff rate being calculated, mathematical fracture closure model was established according to theory of fluid mechanics and rock mechanics and material equilibrium. Meanwhile, the numerical methods of simulating wellhead pressure drop and calculating fracture closure time were given. Numerical simulations showed that fractures in low permeability reservoirs should be closed by using aggressive closure procedures in order to diminish damages to reservoir. The result of comparing the numerical solution of the model with practical curve of wellhead pressure drop revealed that this pressure drop model could forecast flowback well.Thirdly, the mathematical models of proppant settling in Newtonian fluids and power-law fluids were established with considering the influence of proppant concentration on settling, and the numerical method of calculating distance of proppant settling in hydraulic fracture was given. On the other hand, results of experimental investigations on propppant flowback phenomenon were summarizedand factors affecting critical flowback rate was emphatically analyzed.Finally, based on those conclusions of the study on the laws of fracturing fluid leakoff, fracture closure, proppant settling, proppant flowback and so on, the methods and rules used to design flowback procedures in terms of different conditions of reservoirs and parameters of hydraulic fracturing were presented. Moreover, the software on forecast of parameters with regard to flowback was developed and its purpose was to provide guidance for flowback controlling by virtue of optimizing those flowback parameters.